Project Summary Hookah (i.e., shisha, narghile, waterpipe) is one of the most commonly used tobacco products among US young adults. Hookah tobacco exposes users to high levels of carcinogens and other harmful toxicants, can lead to addiction, and is associated with cancer and other negative health outcomes. Despite this evidence, young adults do not view hookah as harmful or addictive, believe intermittent social use poses few risks, and appealing product features like flavorings contribute to growing use in this population. The Food and Drug Administration's (FDA) deeming rule positions FDA to engage in public education messaging targeting young adults to correct pervasive misbeliefs about hookah and offset the influence of appealing flavorings. Our research indicates messages conveying harms and addictiveness of hookah can motivate cessation in young adult hookah users and reduce susceptible non-users' intentions to use hookah. Beyond this research, there is unfortunately little evidence on how to optimally design hookah public education messages targeting young adults. This study aims to identify optimal hookah public education message content within 4 themes (Health Harms, Addiction, Social Use, Flavoring) and test message effects through a rigorous, multi-method approach. We will deploy psychophysiological measures capturing cognitive processing, appetitive and aversive emotional response, and attention in a locally-recruited sample to identify message content that optimally engages physiological processes that are crucial to message efficacy. We will also test the messages in a large crowdsourced sample using valid self-report measures of message engagement, receptivity, emotional response, and hookah beliefs. This process will identify optimal message content within each of the 4 themes. We will then test message effects prospectively in young adult hookah tobacco users and non-users who are susceptible to hookah use in a randomized trial. Participants recruited from a national consumer research panel will complete baseline measures and be randomized to two arms: hookah public education messaging or control. Participants in the messaging arm will receive weekly hookah education messages for 4 weeks and will complete brief assessments of message receptivity, engagement, and hookah-related outcomes. Control arm participants will receive weekly messages unrelated to tobacco, and will complete the same weekly measures. Behavioral outcomes and hypothesized mediators of message effects will be re-assessed 1-, 3-, and 6-months after the exposure period. This study will fill a critical gap in the tobacco regulatory research by demonstrating how to optimally design hookah tobacco public education messages targeting young adults and testing their efficacy prospectively. This evidence is crucial to inform FDA public education activities, and our rigorous multi-method approach provides data to meet FDA's mandate to shape regulatory measures based on their effects on tobacco users and non-users, and by understanding their effect on individual- and population-level outcomes.